The present invention relates to a developing unit for use in an image forming apparatus, such as a printer, a facsimile machine or a copying machine, which forms an image by using an electrophotography technique.
In general, an image forming apparatus using an electrophotography technique includes a photosensitive member having a photosensitive layer on the outer surface thereof, a charging device for uniformly electrically charging the outer surface of the photosensitive member, an exposing device for selectively exposing the outer surface, which has been uniformly electrostatically charged by the charging device, so as to form an electrostatic latent image, a developing device for supplying toner serving as a developer to the electrostatic latent image formed by the exposing device so as to form a visible image (a toner image) and a transfer device for transferring the toner image formed by the developing device to a transfer medium, such as paper.
A conventional developing unit includes a case for accommodating toner; a developing roller rotatably supported by the case by dint of a shaft; a supply roller rotatably supported by the case by dint of a shaft and formed of an elastic member arranged to be pressed against the surface of the developing roller so as to supply toner to the surface of the developing roller; and a conveying fin rotatably supported by the case by dint of a shaft in such a manner as to convey toner to the surface of the supply roller, wherein the elements are sequentially disposed in a horizontal direction.
A conventional developing unit also includes a restraining blade for restraining a quantity of toner on the developing roller, as well as a sealing member disposed on the sides of the developing roller and the supply roller so as to prevent leakage of toner through their shaft portions to the outside of the case.
The conventional developing unit uses a known contact development method in which the developing roller and the photosensitive member are disposed adjacently or in contact with each other. In the contact method, an edge of the end portion of the surface of the developing roller for conveying toner comes in direct contact with the photosensitive member, which can undesirably damage the photosensitive member. If the developing roller is made of a solid material, such as metal, even a non-contact development method encounters damage of the photosensitive member attributable to sliding and friction depending upon the accuracy in the deflection of the developing roller and that of the photosensitive member, as well as the contact development method.
Further, the conventional developing unit, has an elastic conveying member made of an expanded material or the like. Therefore, toner is continuously conveyed from the conveying member to the supply roller portion. As a result, the quantity of toner which is conveyed is larger than the quantity of toner which has been consumed by the developing roller in forming images. Thus, toner is compressed in the supply roller portion and the developing roller portion. If such compression is continued, the pressure in the case at the positions near the developing roller is excessively raised by the restraining blade to appropriately restrain toner on the developing roller. The excessive conveyance of toner from the restraining blade changes the density of a formed image and causes toner to be leaked. If color toner having unsatisfactory fluidity as compared with that of black toner is used, the above-mentioned compression becomes more critical because color toner having poor fluidity is continuously conveyed in the developing unit, and excess toner cannot be returned from the supply roller.
If the conventional developing unit performs a development process with dense toner having poor fluidity, the conveying fin generates a great rotational load which undesirably changes the necessary torque thereby causing rotation of the motor which drives the conveying fin. As a result, jitters appear in the formed image.
Moreover, the conventional developing unit suffers the problem that filming of the electrified members (such as the developing roller and the restraining blade), easily occurs because of mechanical contact and friction when the development process is performed using dense toner containing a large quantity of pigment. If filming of, for example, the pigment having the same polarity as that of toner, occurs with the electrified member, the electrification characteristic of toner deteriorates and becomes instable. When the electrification characteristic of the toner is instable, the density of the formed image is lowered and the toner supply characteristic deteriorates.
The conventional developing unit also has the toner seal disposed on the outer surface ends of the developing roller, thus toner in the sealing portion of the developing roller is not covered. Therefore, in the known contact development method, the photosensitive member is damaged because the sealing portion directly slides on the photosensitive member. When the developing roller is made of a solid material, such as metal, even a non-contact development method encounters damage of the photosensitive member attributable to sliding and abrasion depending upon the accuracy in the deflection of the developing roller and that of the photosensitive member, as well as the contact development method.
One problem encountered by the conventional developing unit is that the portion of the elastic supply roller pressed against the developing roller is dented, causing end portions of the supply roller to project sideways. The projecting portions then undesirably engage the sealing member, thereby requiring an excessively large drive torque in order to rotate the supply roller.
Another problem with the foregoing conventional developing unit arises from the hardening of the supply roller. The supply roller of the foregoing conventional developing unit is comprised of an elastic member, which is usually an expanded material having cells formed in the surface thereof. The portion of the supply roller which contacts with the developing roller encounters introduction of toner into the expanded material through the cells formed in the surface of the expanded material. As a result, the hardness of the expanded material is increased excessively after being used for a long time. The problem associated with the increase in hardness of the supply roller is that great torque is required to rotate the supply roller. To solve this problem, a supply roller comprising a closed-cell expanded material has been suggested. However, in recent years, the average particle size of toner has been reduced to 9 xcexcm, and toner having such small particle size easily clogs in the cells formed in the surface of the conventional supply roller. The elastic characteristic of the supply roller thus deteriorates in a relatively short time.
Another problem in the foregoing conventional developing unit is that, if the rotational speed of the developing roller is increased to quickly form images, or if the fluidity of toner is increased to maintain the required toner supply characteristic, then toner is introduced into the end surface (the side surface) of the developing roller when the developing roller is rotated. As a result, toner leaks from the end surface of the developing roller into the image forming portion thus causing the inside portion of the image forming apparatus to be contaminated. Another problem associated with increased rotational speed of the developing roller and increased fluidity of toner is the leakage of toner from the lower surface of the developing roller during rotation of the developing roller. This also contaminates the inside portion of the image forming apparatus. When the image forming apparatus is contaminated in either above manner, it produces a defective image.
Another problem in the above-mentioned conventional developing unit is that an edge of the end portion of the surface of the developing roller comes in direct contact with the photosensitive member thereby causing damage to the photosensitive member.
The photosensitive member is also damaged by direct sliding contact with the sealing portion of the developing roller when a contact development method is used. In a contact development method, the developing roller and the photosensitive member are disposed adjacently or are brought into contact with each other. The toner seal for the developing roller of the conventional developing unit is disposed at the outer surface of the ends of the developing roller which leaves toner in the sealing portion of the developing roller not covered and which allows the seal to contact the photosensitive member.
In either case, if the developing roller is made of a solid material, such as metal, even a non-contact development method causes damage to the photosensitive member attributable to sliding and friction depending upon the accuracy in the deflection of the developing roller and that of the photosensitive member.
Another problem in the above-mentioned conventional developing unit arises in the conveyance of toner. Toner in the case is sequentially conveyed by the conveying member to the supply roller portion, and then from the supply roller to the developing roller portion. In a developing unit having an elastic conveying member made of an expanded material or the like, conveyance of toner from the conveying member to the supply roller portion is continuous. As a result, the quantity of toner which is continuously conveyed is larger than the quantity of toner which has been consumed for forming images by the developing roller. The excess toner in the vicinities of the supply roller portion and the developing roller portion causes a state of compression. If the foregoing compression is continued, pressure in the case is raised at positions near the developing roller. As a result, toner on the developing roller cannot be restrained by the restraining blade, which leads to an excess conveyance of toner that causes undesirable changes in the density of the image and also causes undesirable toner leaks.
The above problems are exacerbated when color toner is used. Color toner generally has fluidity inferior to that of black toner. Specifically, color toner contains resin of a type having a multiplicity of low-molecular-weight components in order to realize color transmissivity and a dispersant for uniformly dispersing color pigment. The foregoing components deteriorate the fluidity of the toner. If color toner having poor fluidity is continuously conveyed in the above-mentioned developing unit, excess toner cannot be returned from the supply roller, which makes the state of compression more critical.
Additional problems with the conventional developing unit arise when color toner is used. The foregoing electrophotographic process using color toner is performed in such a manner that four developing units, for forming yellow, magenta, cyan and black images, are disposed in the apparatus. The use of four developing units increases the size of the apparatus so that it is much larger than an apparatus for forming a monochrome image. To decrease the size of the apparatus, the density of pigment in each toner particle must be increased to reproduce a required image density with a smaller quantity of toner. By using a smaller quantity of toner, the capacity of the toner case can be reduced.
However, if the pigment component in the toner is increased, the fluidity of the toner generally deteriorates, thus causing a great rotational load on the conveying fin. The increased load on the conveying fin undesirably changes the necessary torque to drive the conveying fin which in turn causes undesirable changes in the rotation of the motor which drives the conveying fin. As a result, jitters appear in the formed image.
Increasing the pigment component in the toner also raises the area ratio of the pigment component on the surface of the matrices of toner particles in general. Toner must have a certain polarity and be frictionally electrified by an electrified member having a polarity opposite to the polarity of toner (such as a developing roller or a restraining blade) so that the electrification of toner is stabilized. If toner particles have pigment in a large quantity on their surfaces, the electrified members (such as the developing roller and the restraining blade) easily encounter filming attributable to mechanical contact and sliding. If the electrified member having the same polarity as that of toner encounters filming, the electrification characteristic of toner deteriorates and becomes instable. As a result the density of the formed image is lowered and the toner supply characteristic deteriorates.
Further problems arise in the above-mentioned conventional developing unit when trying to reduce its size. The center or rotation of the conveying fin in the conventional developing unit is disposed lower than the center of rotation of the supply roller in an attempt to reduce the thickness of the developing unit by efficiently creating a space for accommodating toner. However, such an arrangement suffers the problem that the conveying fin scrapes insufficient toner up to the surface of the toner supply roller thereby causing an undesirably low density in the formed image.
One object of the present invention is to provide a developing unit which is capable of reducing the torque necessary to drive the supply roller. A further object of the present invention is to provide a developing unit capable of reducing torque required to rotate the supply roller. It is also an object of the present invention is to provide a developing unit which prevents an increase in the torque required to rotate the developing roller and prevents leakage of toner. Yet another object of the present invention is to provide a developing unit which is capable of conveying toner without change in torque even if toner having poor fluidity, such as toner having a high density, is used.
Another object of the present invention is to provide a developing unit capable of protecting a photosensitive member from being damaged and exhibiting an excellent sealing characteristic.
Another object of the present invention is to provide a developing unit having a seal which does not damage the photosensitive member.
Still another object of the present invention is to provide a developing unit capable of preventing compression of toner in a case wherein the compression is due to conveyance performed by a conveying member. It is also an object of the present invention to prevent excess conveyance of toner.
A further object of the present invention is to provide a developing unit which is capable of ensuring that the conveying fin satisfactorily conveys toner.
Another object of the present invention is to provide a developing unit which is capable of preventing the occurrence of filming to the developing roller and the restraining blade even if dense toner is used.
To achieve the above-mentioned objects, the developing unit of the present invention includes a case for accommodating toner; a developing roller rotatably supported by the case by dint of a shaft; and a supply roller rotatably supported by the case by dint of a shaft and formed of an elastic member arranged to be pressed against the surface of the developing roller in such a manner as to supply toner to the surface of the developing roller. In the present invention, the elastic member which forms the supply roller is an expanded material having a ratio of open cells of 30% or higher, the depth of engagement of the supply roller to the developing roller is 0.4 mm or smaller, and the toner has a shape factor SF-1 of 150 or smaller and a shape factor SF-2 of 140 or smaller. As a result, even if toner is introduced into the cells formed in the surface of the supply roller, the cells in the surface are not clogged as has been experienced with the closed-cell expanded material. Moreover, toner introduced into the cells can easily be discharged from the cells. Therefore, the undesirable rise in the hardness of the supply roller over time, as has been experienced with the conventional structure, is prevented. By preventing the hardening of the supply roller, the torque required to rotate the supply roller can be reduced.
Additionally, corner portions of the supply roller can be chamfered. When the corner portions of the supply roller are chamfered, even if the elastic portion of the supply roller pressed against the developing roller is dented, the projecting portions do not project sideways over the side surface of the supply roller. Since the projecting portions are not engaged with the sealing member as has been experienced with the conventional technique, the drive torque required to rotate the supply roller is reduced.
To further achieve the above-mentioned objects, the developing unit of the present invention includes a developing roller having a chamfered end portion on the surface for conveying toner. Therefore, the end portion of the developing roller does not come in contact with the photosensitive member. Thus any damage to the photosensitive member, which occurs due to contact and sliding of the edge of the end portion of the developing roller, is prevented. Since the quantity of elastic displacement of the sealing member is made lager than the quantity of chamfering of the end portion of the developing roller, the sealing member is able to flexibly follow the chamfered portion of the developing roller. As a result, leakage of toner from the chamfered portion is prevented. Thus, the characteristic for sealing toner can further be improved.
To further achieve the above-mentioned objects, the developing unit of the present invention also includes a restraining blade for restraining a quantity of toner on the developing roller wherein the restraining blade is at least longer than a surface of the developing roller for conveying toner. Therefore, toner on the overall surface of the developing roller is uniformly restrained by the restraining blade thereby preventing any defect of the toner seal which is caused from toner excessively conveyed from the surface of the developing roller or free toner which appears attributable to an excess conveyance of toner. Moreover, because toner is uniformly formed over the surface of the developing roller for conveying toner, therefore the toner serves as a lubricant between the photosensitive member and the developing roller. As a result, damage of the photosensitive member is reduced.
To further achieve the objects of the invention, the developing unit of the present invention also includes a sealing member disposed on the side ends of the developing roller and the supply roller so as to prevent leakage of toner from the shaft portion. Since the developing unit according to the present invention has the seals disposed at the ends of the developing roller, introduction of toner from the surface of the developing roller for conveying toner into the side end of the developing roller is prevented. The sealing portion of the developing roller does not damage the photosensitive member as has been experienced with the conventional structure and the present invention achieves excellent toner sealing.
Further, a quantity of elastic displacement of the sealing member is larger than a quantity of chamfering of the end portion of the developing roller. Because the quantity of elastic deformation of the sealing member is larger than the difference between the length of the restraining blade and the length of the surface of the developing roller, the sealing member flexibly follows a stepped portion generated by the difference in the length between the developing roller and the restraining blade. Thus, leakage of toner from the stepped portion is prevented, and the characteristic for sealing toner is further improved.
The surface roughness of the side surface of the developing roller is 0.5 xcexcm or smaller in Rmax. If the surface roughness of the side surface of the developing roller exceeds 0.5 xcexcm in Rmax, toner is undesirably held on the side surface of the developing roller, and is undesirably conveyed thereby when the developing roller is rotated. Thus, toner leaks to the outside portion of the developing unit.
Because the side surfaces of the developing roller of the present invention have a surface roughness of 0.5 xcexcm or smaller in Rmax, toner is not held thereon and is not conveyed thereby when the developing roller is rotated. Therefore, introduction of toner between the side surface of the developing roller and the sealing member as has been experienced with the conventional structure is prevented, which in turn prevents leakage of toner to the outside portion of the developing unit.
Moreover, the sealing member is in the form of a film which is allowed to abut against the developing roller, and a quantity xcex4 of displacement of the sealing member occurring attributable to the abutment of the sealing member against the developing roller satisfies 0.1 mm less than xcex4 less than 0.8 mm. Therefore, toner is not scraped down by the sealing member when the developing roller holding toner passes through the developing position and is then recovered in the developing unit.
Since the quantity xcex4 of displacement of the sealing member satisfies 0.1 mm less than xcex4, the sealing member is always brought into contact with the overall region of the developing roller in the lengthwise direction even if the deflection of the developing roller is tens of xcexcm. Thus, leakage of toner to the outside portion of the developing unit is prevented.
Because the quantity xcex4 of displacement of the sealing member satisfies xcex4 less than 0.8 mm, the torque required to rotate the developing roller is not substantially increased when the sealing member is brought into contact with the developing roller. Therefore, leakage of toner is prevented even if images are formed quickly as in the conventional structure. Moreover, increase in the torque required for rotating the developing roller is prevented.
To achieve the above desired displacement of the sealing member, the thickness t of the sealing member satisfies 50 xcexcm less than t less than 500 xcexcm. Since the thickness t of the sealing member satisfies 50 xcexcm less than t, satisfactory printing durability can be realized. After a multiplicity of sheets have been printed, the force of contact of the sealing member with the developing roller is not substantially changed, and thus, toner is not leaked to the outside portion of the developing unit.
Since the thickness t of the sealing member satisfies t less than 500 xcexcm, the torque required to rotate the developing roller is not substantially enlarged when the sealing member is brought into contact with the developing roller.
To further achieve the above mentioned objects, a developing unit of the present invention also includes a conveying member which comprises at least one or more conveying fins each having a fin shape and being rotatably supported by the case by dint of a shaft in such a manner as to convey toner to the surface of the supply roller. Thus, toner accommodated in the case is conveyed to the surface of the supply roller by dint of mechanical conveying force generated when the conveying fins are rotated. The conveying fins are lightly in contact with the case therefore toner placed on the conveying fins and that placed between the conveying fins and the case is conveyed to the surface of the supply roller.
Since each conveying fin has the fin-type shape as described above, toner is continuously supplied. As an alternative to this, toner can be intermittently supplied to the supply roller, which would prevent an excess conveyance of toner.
The conveying fin is disposed in such a manner that the angle xcex8 made between the line connecting the center of rotation of the supply roller to the center of rotation of the conveying fin and a horizontal line is in a range from xe2x88x9220 degrees to +75 degrees, wherein a clockwise direction of rotation has positive values with respect to a horizontal direction. Therefore, toner on the conveying fin and that between the conveying fin and the case can easily be scraped up to the surface of the supply roller.
If angle xcex8 is xe2x88x9220 degrees or smaller, toner on the conveying fin and that placed between the conveying fin and the case cannot be scraped up to the surface of the supply roller. Even if toner is scraped up to the surface of the supply roller, if the structure has no support roller for supporting toner in the direction of gravity then toner on the supply roller will drop off. Thus, toner cannot satisfactorily be conveyed.
If the angle xcex8 is 75 degrees or larger, toner on the conveying fin and toner placed between the conveying fin and the case are excessively conveyed to the supply roller portion. Thus, toner is compressed excessively among the supply roller, the developing roller and the restraining blade. As a result, the restraining blade cannot appropriately restrain toner on the developing roller and toner leaks from the restraining portion.
The developing unit of the present invention is thus able to prevent the defect in conveyance which takes place with the conveying fin of the conventional structure.
Alternatively, the angle xcex8 made between the line connecting the center of rotation of the supply roller to the center of rotation of the conveying fin and the horizontal line can be in a range from xe2x88x9220 degrees to 0 degrees. In such an arrangement, the conveying fin is supported so that it is displaced and brought into contact with the case, and so that it is rapidly displaced and released from the case at a portion adjacent to the supply roller. Therefore, a larger force is produced for elastically discharging toner on the conveying fin and toner placed between the conveying fin and the case.
Therefore, a large quantity of toner which must be conveyed can be elastically discharged to the surface of the supply roller. As a result, the efficiency of conveying toner to the surface of the supply roller is improved.
In the developing unit of the present invention when the angle xcex8 made between the line connecting the center of rotation of the supply roller to the center of rotation of the conveying fin and the horizontal line is in a range from xe2x88x9220 degrees to 0 degree, an apparent density (hereinafter called an A.D) of toner used is 0.3 g/cc or higher to improve the toner conveying the efficiency. As a result of using toner of the foregoing type, toner is easily scraped up by the conveying fin and conveyed from the conveying fin to the supply roller. Thus, toner does not easily drop from the surface of the supply roller, and the toner conveying efficiency is further improved.
If the A.D of toner is lower than 0.3 g/cc, the toner has poor fluidity and cannot be easily scraped up by the conveying fin to the surface of the supply roller.
If the A.D of toner is 0.5 g/cc or higher, excessive fluidity is realized and toner scraped up by the conveying fin to the surface of the supply roller easily comes off the supply roller.
The developing roller, the supply roller and the conveying fins are sequentially disposed in a horizontal direction and the number of revolutions of the conveying fins is not less than {fraction (1/50)} of the number of revolutions of the supply roller nor more than {fraction (1/20)} of the same. Since the conveying fins are structured to rotate in such a manner, excessive conveyance of toner to the supply roller portion by the conveying fins is further prevented.
If the number of revolutions of the conveying fins is larger than {fraction (1/20)} the number of revolutions of the supply roller, the amount of toner conveyed by the conveying fins is in excess of that which is consumed by the developing roller and the supply roller. Thus, toner is brought to the compressed state. In the compressed state, toner is moved before the excess portion of toner conveyed is returned to the conveying fins, thus the state of compression becomes more critical.
If the number of revolutions of the conveying fins is smaller than {fraction (1/50)}, the number of revolutions of the supply roller, the quantity of toner which is conveyed by the conveying fins is insufficient to compensate the quantity of toner which is consumed by the developing roller and supply roller portions. The developing unit of the present invention thus prevents excessive compression of toner and also prevents defects in conveyance caused by the conveying member.
In another embodiment of the present invention, at least two or more conveying fins are rotatably supported by the case by dint of a shaft, and the phases of rotation of the conveying fins are different from one another. Further, the number of revolutions of a conveying fin nearest the supply roller is larger than the number of revolutions of any other conveying fin. Therefore, toner in the case is slowly conveyed to the supply roller portion. As a result, compression of toner in the supply roller portion is prevented. Since the conveying fin nearest the supply roller has a higher number of revolutions, the insufficient conveyance of toner can be prevented and the developing unit of the present invention is further able to prevent compression of toner by the conveying fins.
Specifically, the fins are formed by thin flexible plates (in the form of sheets). The fins are rapidly displaced and released from the wall surface of the case at a position adjacent to the supply roller. More specifically, when the fin is separated from the wall surface of the toner reservation portion, toner conveyed while being held in a wedge-shape space formed between the fin and the case is elastically discharged by the elasticity of the fins. To achieve this elastic discharge, the quantity of displacement between the fin and the wall surface is enlarged. The force for discharging toner depends on the rigidity (the elasticity) of the fin.
Since the phases of rotation of the conveying fins are different from one another, the change in the load occurring attributable to the elastic discharge of toner performed by the conveying fin is dispersed. Thus, even if dense toner having poor fluidity is used, the load of the conveying fins is uniformly distributed and undesirable change in the torque in the apparatus can be prevented.
The toner of the present invention contains pigment by 5 wt % or more, and a ratio of a toner additive having a small diameter covering the surfaces of matrices of toner is 100% or higher. Therefore, even if a large quantity of pigment exists on the surfaces of the matrices of the toner, the additive having a relatively small diameter surrounds the pigment. Consequently, the pigment existing on the surfaces of the matrices does not come in contact with an electrified member (such as the developing roller or the restraining blade) and does not slide on the same. Thus, the electrified member is free from filming. Because the additive having a relatively small diameter is usually made of an inorganic material having high hardness, the additive does not allow the electrified member to easily encounter filming even if mechanical contact and sliding take place. Thus, filming of the pigment of toner to the electrified members is prevented.
The size of a record toner additive having a large diameter is not smaller than xc2xd of the diameter of the pigment. Thus, again even if a large quantity of the pigment exists on the surfaces of the matrices of toner particles, the additive having a relatively large diameter projects over the surfaces of the matrices of the particles covers the surfaces of the pigment. Therefore, the pigment existing on the surface of the matrices of the toner particles does not come in contact with the electrified members, such as the developing roller and the restraining blade. Thus, filming of the electrified members is prevented. The reason xc2xd is employed is because the maximum value of the quantity of projection of the pigment over the surfaces of the matrices of the particles is xc2xd. Pigment projecting by the quantity exceeding xc2xdis separated during the process for manufacturing toner and thus does not exist. Therefore, contact of the pigment with the electrified members is prevented if the particle size of the additive covering the pigment is xc2xd or larger.
When the size of the large diameter toner additive is larger than xc2xd the diameter of the pigment, the ratio of the additive covering the surfaces of matrices of particles is not larger than 10%. Therefore, the additive is able to uniformly and fully cover the pigment even if a large quantity of the pigment exists on the surfaces of the matrices of toner particles, and filming to the electrified members is further prevented.
Moreover, an apparent density (hereinafter called an A.D) of the-toner is 0.3 g/cc or higher.
Accordingly, in the developing unit of the present invention, torque required to rotate the supply roller is reduced, and toner is prevented from leaking out of the developing unit. Even toner, such as dense toner, having poor fluidity is conveyed without any change in the torque, and an image free from jitters is formed. Also, the developing unit prevents leakage of toner even after a multiplicity of images has been printed. Further in the developing unit, the photosensitive member is not damaged by the end portion of the surface of the developing roller and the sealing characteristic is improved. Moreover, in the developing unit, undesirable compression of toner in the case which occurs due to conveyance performed by the conveying member is prevented. Thus, change in the density of a formed image is prevented. Further, the developing unit prevents filming of the pigment of toner to the electrified members (such as the developing roller and the restraining blade) even if toner, such as dense toner, containing a large quantity of pigment is employed. Finally, in the developing unit of the present invention, the conveying fin is able to efficiently convey toner even if the developing roller, the supply roller, and the conveying fin are sequentially disposed in the horizontal direction. Thus, the densities of formed images are stabilized.